PURPOSE: To improve dielectric strength by providing the buried layer of an I2L transistor to the boundary part between the 1st and 2nd epitaxial stacked layers grown on a semiconductor substrate and the buried layer of a bipolar transistor to the boundary part between the substrate and 1st epitaxial layer.

CONSTITUTION: On P-type semiconductor substrate 1, N+-type 1st buried layer 2 is diffusion-formed as the collector of a bipolar transistor, and on the entire surface, N- type 1st epitaxial layer 3 is grown while buried layer 2 is diffused again at the same time to flow partially in layer 3. Next, N+-type 2nd buried layer 2a as the emitter of an I2L transistor is diffusion-formed and on it, N-type 2nd epitaxial layer 3a is grown, but buried layer 2a flows as well. In an ordinary way afterward, P-type isolation region 4, P-type injector 5, P-type 1st and 3rd regions 6 and 8, and N+- type 2nd, 4th and 5th regions 7, 9 and 10 are formed and an internal wiring is provided, thereby obtaining an IC.